A gallon of latex paint can cover 400 square feet.  Maria has a rectangular room whose dimensions are 11 feet by 21 ​feet, with 10​-foot ceilings.  How many gallon containers of paint does she need to buy to paint two coats on each​ wall?

To solve this problem, we need to calculate the total area of the walls that Maria needs to paint, then determine how many gallons of paint will be required to cover that area with two coats.

### Step 1: Calculate the area of each wall
Maria's room is rectangular, with dimensions of 11 feet by 21 feet and 10-foot ceilings. The room has four walls, two of each dimension.

- **Area of the two shorter walls (11 feet wide, 10 feet tall):**
  \[
  \text{Area of one short wall} = 11 \times 10 = 110 \text{ square feet}
  \]
  \[
  \text{Area of two short walls} = 2 \times 110 = 220 \text{ square feet}
  \]

- **Area of the two longer walls (21 feet wide, 10 feet tall):**
  \[
  \text{Area of one long wall} = 21 \times 10 = 210 \text{ square feet}
  \]
  \[
  \text{Area of two long walls} = 2 \times 210 = 420 \text{ square feet}
  \]

### Step 2: Calculate the total wall area
Add the areas of all four walls together:
\[
\text{Total wall area} = 220 + 420 = 640 \text{ square feet}
\]

### Step 3: Calculate the area for two coats of paint
Since Maria wants to apply two coats of paint:
\[
\text{Total area for two coats} = 2 \times 640 = 1280 \text{ square feet}
\]

### Step 4: Determine the number of gallons needed
One gallon of paint covers 400 square feet, so:
\[
\text{Number of gallons needed} = \frac{1280}{400} = 3.2 \text{ gallons}
\]

Since Maria can't buy a fraction of a gallon, she will need to purchase **4 gallons** of paint.

### Summary
Maria needs to buy 4 gallon containers of paint to cover the walls with two coats.

Would you like more details or have any questions?

Here are 5 related questions:
1. How would the number of gallons change if Maria wanted to paint the ceiling as well?
2. What if the paint covered only 350 square feet per gallon instead of 400?
3. How many square feet of wall could be painted with just one coat if Maria bought 5 gallons?
4. How would the calculation change if the room had a door or windows?
5. What would be the cost if each gallon of paint costs $25?

**Tip:** Always round up when calculating quantities like paint, since you can't purchase a fraction of a container.

Discover how much paint Maria needs to buy to paint her rectangular room, 11 feet by 21 feet, with 10-foot ceilings. The solution involves calculating wall areas, determining paint coverage, and ensuring two coats on each wall. Suitable for students in Grades 6-8.

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